The use of electromagnetic radiation to investigate samples is well known. Reflectometer, spectrophotometer, ellipsometer, and polarimeter systems, for instance, direct a beam of electromagnetic radiation to interact therewith, (in reflection and/or transmission), a sample, which beam then enters a detector. Detected changes in Intensity (in reflectometer and spectrophotometer systems), and Polarization State (in ellipsometer, and polarimeter systems), as a result of said interaction provide insight to properties of the sample. Properties such as absorption constant, ellipsometric Psi and Delta etc. are evaluated, typically by performing a mathematical regression of accumulated data onto a mathematical model of the sample.
It is always of benefit to investigate a sample with multiple angles-of-incidence of the beam to a sample surface, and with as many wavelengths as possible. In the later is found a major innovation of the present invention which identifies and applies as a source of a beam of electromagnetic radiation a supercontinuum laser. Briefly, as described in an Article titled “Supercontinuum”, said terminology “supercontinuum laser” refers to a source of electromagnetic radiation that results from interaction of a typically pulsed laser and many non-linear processes to cause extensive spectral broadening. (Note, “extensive” means beyond a single wavelength, and in the present invention it can be interpreted to mean a spectrum between about 400 to about 2500 nm. Non-linear processes include self-phase modulation, four-wave mixing, soliton dynamics and cross-phase modulation. (The term “soliton” refers to relatively permanent localized waveforms that are formed from dispersion and non-linearity effects. For instance, the refractive index of a material at a given frequency depends on the amplitude of electromagnetic radiation applied, (ie. the non-linear Kerr effect). If a pulse has the right shape said Kerr effect will exactly cancel dispersion in the material and the pulse's shape will not change over time, thereby forming a “soliton”. Also, the term “four-wave mixing” refers to the effect wherein interaction between two or three wavelengths produces additional new wavelengths, (eg. a non-linear interaction of two different wavelength beams affect a third wavelength beam such that a fourth wavelength beam is generated). Continuing, said “Supercontinuum” article provides that the two most important regimes are soliton fusion and modulation instability, and that a third regime involves pumping in the normal, (ie. refractive index decreases with increase in wavelength), dispersion region. Soliton fusion involves application of a high powered femto-second pulse being launched into highly non-linear photonic crystal, or other highly non-linear fiber, where the femto-second pulse can be considered a high order soliton which rapidly broadens and then fissions into fundamental solitons. The resulting fundamental solitons then undergo intra-Raman scattering and shift to longer wavelengths, (known as soliton a self-frequency shift), which generates a long wavelength side of a continuum. A sorter wavelength side of a continuum is formed when longer soliton self frequencies as dictated by group velocity matching conditions via a soliton trapping mechanism. This approach to producing a supercontinuum is characterized by the production of high temporal coherence. A disadvantage of this approach is that high average powers in the produced continuum are not achievable. The modulation instability regime involves the breakup of a continuous or quasi-continuous wave field. The long-wavelength side of the continuum formed in this regime is generated by intra-pulse Raman scattering and self frequency sifting. The short wavelength side is generally attributed to soliton fission and dispersive wave generation. The normal dispersion range pumping approach, where pulses are short enough, is attributed to self-modulation which can lead to significant broadening which is temporarily coherent. Where pulses are not ultra-short then stimulated Raman scattering tends to dominate and typically a series of cascaded discrete Stokes lines will appear until the zero dispersion wavelength is reached. At this point a soliton Raman continuum can form. Pumping in the anamolous range is more efficient for continuum generation. For insight it is noted that the normal pumping range dispersion is that in which an increase in wavelength leads to a decrease in refractive index. This relationship is the opposite in the anamolous range. For an increase in wavelength the refractive index increases. For additional insight, Raman scattering of photons is inelastic, and mediated by an exciton. Only 1 in about 10 million photons that are scattered from a molecule or atom are scattered by the inelastic Raman effect, but those that are present at a different frequency than was the initial photon, and the wavelength of the scattered photon is usually longer, therefore the scattering molecule, for instance, can be excited to a higher energy level by the interaction.
While the formation of a supercontinuum laser spectrum is the result of many complex non-linear effects, such need not concern us as regards the present invention which is not dependent on how a supercontinuum is produced, only that it is and is applied in such as a reflectometer, spectrophotometer, ellipsometer or polarimeter.
An additional effect that must also be considered is the result of coherence that results in a supercontinuum range of wavelengths. When coherence is present it is very well known that interference effects result based on differences in phase between interacting waves. When lasers are involved the effects of interference is often referred to as “Speckle” and importantly this leads to time varying beam profiles. To be conveniently applied in a system such as a reflectometer, spectrophotometer, ellipsometer or polarimeter the complexity entered by such spatial and temporal fluctuations must be reduced. The present invention therefore applies speckle reducers, such as Optotune Elastoactive Polymer and Reflective Force LSR's.
Turning now to Identified Patent literature that has relevance regarding Supercontiuum Laser Sources, the following were identified:                U.S. Pat. No. 8,422,519 to Knight et al.;        U.S. Pat. No. 8,718,104 to Clowes et al.; and        Published App. No. 2014/0233091 by Clowes et al.        
The present invention also involves application of improved detector systems. In that light, it is emphasized that it is well known to apply detectors of electromagnetic radiation in, for instance, ellipsometry. And it is known to use beam splitters to direct portions of beams into different detectors which can be optimized to respond to different wavelength ranges. A Patent to Herzinger et al. U.S. Pat. No. 8,169,611, for instance shows such an arrangement in FIG. 1a thereof. Many other references showing similar application of beam splitters in a similar manner exist. Also known are monochromater systems that utilizes a sequence of gratings with which a beam of spectroscopic electromagnetic radiation sequentially interacts to select a desired wavelength. FIG. 9 in a Patent to Liphardt et al. U.S. Pat. No. 7,345,762 provides an example that shows such an arrangement, and FIG. 2 in said Patent 762 also demonstrates an ellipsometer or polarimeter system.
Another known Patent is U.S. Pat. No. 6,104,488 to LeVan. This Patent is focused on providing high single grating efficiency, with different orders of wavelengths being produced thereby detected by a single detector.
An article titled “A New Spectrometer Using Multiple Gratings With A Two-Dimensional Charge-Coupled Diode Array Detector”, Review of Scientific Instruments, Han et al., Vol. 74, No. 6, June 2003, describes a special grating that consists of ranges.
Additional Patent references identified in a computer Search are:                Searching for (Supercontinuum Laser and Ellipsometer) provided five Patents, U.S. Pat. Nos. 9,080,971, 8,873,054, 8,441,639, 8,031,337 and 7,570,358, and six Published Applications, No. 2015/0323316, 2015/0036142, 2013/0222795, 2011/0069312, 2009/0262366 and 2008/0239265; and        Searching of (Supercontinuum & Laser and Ellipsometer and Speckle) provided no Patents and only four Published Applications, Nos. 2015/0058813, 2015/0046121, 2015/0046118 and 2015/0330770.        
Also, known Patents and Published Applications relating to Spekle Reduction are: U.S. Pat. No. 6,895,149 to Jacob et al.; U.S. Pat. No. 7,522,331 to Lapchuk et al.; US 2013/0027673 by Moussa; US 2006/0238743 by Lizotte et al. and US 2013/0010365 by Curtis.
Even in view of the state of the art, there remains need for application of improved sources of electromagnetic radiation and detector systems in reflectometer, spectrophotometer, ellipsometer, and polarimeter systems. Further, there remains need for additional systems directed to optimizing application of a plurality of detectors and/or wavelength dispersing elements which are arranged sequentially, and wherein each follow-on wavelength dispersing element receives a reflected altered spectral content reflected beam of electromagnetic radiation from a preceding wavelength dispersing element, and wherein each wavelength dispersing element produces at least one + or − spectrum of dispersed wavelengths which are directed toward a related detector.